In the following text, the circumferential, axial and radial directions denote a direction tangential to the tread surface of the tire in the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire and a direction perpendicular to the axis of rotation of the tire, respectively. “Radially on the inside or, respectively, radially on the outside” means “closer to or, respectively, further away from the axis of rotation of the tire”. “Axially on the inside or, respectively, axially on the outside” means “closer to or, respectively, further away from the equatorial plane of the tire”, the equatorial plane of the tire being the plane that passes through the middle of the tread surface of the tire and is perpendicular to the axis of rotation of the tire.
In general, a tire comprises a tread, intended to come into contact with the ground via a tread surface, the tread being connected by two sidewalls to two beads, the two beads being intended to provide a mechanical connection between the tire and a rim on which the tire is mounted.
A radial aircraft tire more particularly comprises a radial carcass reinforcement and a crown reinforcement, both as described, for example, in document EP 1381525.
The radial carcass reinforcement is the tire reinforcing structure that connects the two beads of the tire. The radial carcass reinforcement of an aircraft tire generally comprises at least one carcass layer, each carcass layer being made up of reinforcers, usually textile, coated in a polymeric material of the elastomer or elastomer compound type, the reinforcers being mutually parallel and forming, with the circumferential direction, an angle of between 80° and 100°.
The crown reinforcement is the tire reinforcing structure radially on the inside of the tread and at least partially radially on the outside of the radial carcass reinforcement. The crown reinforcement of an aircraft tire generally comprises at least one crown layer, each crown layer being made up of mutually parallel reinforcers coated in a polymeric material of the elastomer or elastomer compound type. Among the crown layers, a distinction is usually made between the working layers that constitute the working reinforcement, usually made up of textile reinforcers, and the protective layers that constitute the protective reinforcement, made up of metal or textile reinforcers and arranged radially on the outside of the working reinforcement. The working reinforcement dictates the overall mechanical behaviour of the crown reinforcement, while the protective reinforcement essentially protects the working layers from attack likely to spread through the tread radially towards the inside of the tire.
The textile reinforcers of the carcass layers and of the crown layers are usually cords made of spun textile filaments, preferably made of aliphatic polyamide or of aromatic polyamide. The mechanical properties under tension, such as the elastic modulus, the elongation at break and the force at break of the textile reinforcers, are measured after prior conditioning. “Prior conditioning” means the storage of the textile reinforcers for at least 24 hours, prior to measurement, in a standard atmosphere in accordance with European Standard DIN EN 20139 (temperature of 20±2° C.; relative humidity of 65±2%). The measurements are taken in the known way using a ZWICK GmbH & Co (Germany) tensile test machine of type 1435 or type 1445. The textile reinforcers are subjected to tension over an initial length of 400 mm at a nominal rate of 200 mm/min. All the results are averaged over 10 measurements.
During the manufacture of an aircraft tire and, more specifically, during the step of laying the working reinforcement, a working layer is usually obtained by circumferential zigzag winding or by a circumferential winding in turns of a strip onto a cylindrical laying surface having as its axis of revolution the axis of rotation of the tire. The strip is generally made up of at least one continuous textile reinforcer coated in an elastomeric compound and, usually, of a juxtaposition of mutually parallel textile reinforcers. Whether produced by circumferential zigzag winding or circumferential winding in turns, the working layer is then made up of the juxtaposition of portions of strip.
Circumferential winding in turns of a strip is understood as meaning a winding of the strip in the circumferential direction and in a helix of radius equal to the radius of the cylindrical laying surface and at a mean angle, with respect to the circumferential direction, of between 0° and 5°. The working layer thus obtained by winding in turns is said to be circumferential because the angle of the textile reinforcers of the strip, one parallel to the next, formed in the equatorial plane with the circumferential direction, is between 0° and 5°.
Circumferential zigzag winding of a strip is understood as meaning winding of the strip in the circumferential direction and with a periodic curve, that is to say a curve formed of periodic waves oscillating between extrema. Winding a strip with a periodic curve means that the mid-line of the strip, equidistant from the edges of the strip, coincides with the periodic curve. During circumferential zigzag winding of a strip, the working layers are laid in pairs, each pair of working layers constituting a working bi-ply. Thus, a working bi-ply is made up, in its main section, that is to say axially inside the axial ends thereof, of two radially superposed working layers. At its axial ends, a working bi-ply generally comprises more than two radially superposed working layers. The axial end portion of a working bi-ply comprising more than two radially superposed working layers is referred to as the axial end overthickness. This axial end overthickness is generated by the crossings of the strip, at the end of the working bi-ply, for each turn of zigzag winding. Such a working reinforcement comprising working bi-plies obtained by circumferential zigzag winding of a strip has been described in documents EP 0540303, EP 0850787, EP 1163120 and EP 1518666.
In the case of circumferential zigzag winding, it is known that the axial end overthicknesses of the working bi-plies are particularly sensitive to the onset of endurance damage, such as cracks which may evolve into significant degradation of the working reinforcement and, therefore, lead to a reduction in the life of the tire. These cracks arise generally at the internal interfaces of the axial end overthickness, that is to say between the radially superposed working layers at said axial end overthickness. Thus, the higher the number of radially superposed working layers at the axial end overthickness, the greater the number of internal interfaces and, therefore, the greater the risk of cracking.